What is Life?
Usually such questions get bogged
down in discussions of what is living and what isn't. I'd like to by-pass that issue for
now and focus on conglomerates of matter that most usually think of as alive - living
things. That means Archaea, Bacteria, and Eukarya, all of which are cells containing DNA.
You, me, your dog and the yeast in your bread are Eukarya - cells with nucleii. Some of us
just happen to be big colonies of cloned Eukarya surrounded by tough bags of dead cells
and reinforcing fibres.
Bacteria and Archaea
are often confused
because they both don't contain their DNA in nucleii, but even though they are related
they're as far apart as we Eukarya are from them. At some point, about 2.7 billion years
ago, an Archaean and a Bacterium merged, diversified and gradually brought more bacteria
into a new symbiosis - this is called the serial endosymbiosis theory of Eukaryotic origins. Today the main signs of this
primordial symbiosis are organelles
- sub-structures within cells -
that seem to be morphologically related to independent bacterial species. In animals the mitochondria, a cell's powerhouses, seem to be former bacteria, while in plants
there are also the chloroplasts which seem to be former algae. These identifications
of previous symbionts seem certain, but several other structures might have bacterial
origins - the undulipodium and the peroxisomes. In a real sense all Eukarya are actually
mixed colonies of Archaea/Bacteria.
Symbiosis isn't the only way that the
various orders of life trade genes. Viriuses are now known to be able to transfer genes between
bacteria and eukarya, a very significant example of which involves the placentas of all
advanced mammals. Thousands of millions of Endogenous Retro-Viruses (ERVs) are produced by the placenta of a forming embryo. Without these viruses the embryo doesn't attach to the mother's uterus successfully, and so is miscarried. Just how mammals and viruses came into this partnership is currently unknown, but it has worked for 150 million years or so.
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